(1) Field of the Invention
The present invention relates to a magnetic carrier for a two-component type developing agent used for developing electrostatic images in an electrophotography method and the like method. More particularly, the invention relates to a carrier for a developing agent which exhibits excellent properties such as reduced weight, durability, resistance against the heat and resistance against the humidity in combination.
(2) Description of the Prior Art
A two-component type developing agent has been widely used for developing electrostatic images in an electrophotographic method and the like method. The two-component type developing agent uses in combination a toner and a carrier which are electrically charged to opposite polarities relative to each other. When the developing is effected, the carrier is held by a developing agent carrier member while the toner migrates on the electrostatic images to from visible images. Magnetic particles such as an iron powder or sintered ferrite particles are widely used as a carrier which, however, are not yet fully satisfactory with respect to controlling the electrically charging property, reduction in the weight and the flowability.
Furthermore, a carrier of small particles obtained by dispersing and granulating a magnetic powder in a resin of a molten binder has been placed in practical use as a magnetic carrier, yet leaving such a problem that the electrically charging property changes greatly with a change in the environment being caused by magnetic particles that are exposed to the surfaces.
Japanese Patent Laid-Open No. 296157/1987 discloses a carrier for a developing agent obtained by melt-spraying a mixture of a resin and a magnetic powder and granulating it through quenching, and Japanese Patent Laid-Open No. 182668/1988 discloses a carrier for a developing agent obtained by providing the carrier surfaces of the above melt-sprayed granules with a coating layer.
When the carrier surfaces are provided with the coating layer as described above, it becomes necessary to heat-treat the coating layer at a temperature as high as more than 100.degree. C. in order to remove solvents remaining in the coating layer as well as to sufficiently polymerize and cure the coating layer. If this heat-treatment is not sufficiently carried out, the electric characteristics of the carrier such as the amount of electric charging and electric resistance become liable to change with the lapse of time and, further the resistance against the humidity and the resistance against solvents become poor.
According to the above prior art, however, since the carrier for the developing agent makes use of a core prepared based on the melt-spraying method, the heat-treatment effected at a temperature higher than 100.degree. C. for the coating layer causes the core to be melted giving rise to the occurrence of blocking. Or, even if the carrier for the developing agent is prepared, its electrical properties are deteriorated due to agitation and abrasion in the developing device.